1. Field of the Invention
This invention relates to concrete and controlled low-strength materials having increased electrical conductivity such that when used in construction, the material is capable of conducting electrical charges such as those resulting from a lightning strike. Further, the concrete and controlled low-strength materials include a spent carbon sorbent, thus providing a means for utilization of a product usually considered a waste product of coal burning power generation.
2. Description of the Related Art
It is known that fly ash can be incorporated into concrete. See, for example, U.S. Pat. Nos. 6,821,336, 6,461,424, 4,116,705, 4,268,316, 5,520,730, 5,853,475, 5,346,012, 5,490,889, 5,374,308, 4,230,568, 4,050,261 and 4,210,457; European patent application EP 744386; Davis et al., “Weathering Resistance of Concretes Containing Fly-Ash Cements”, Journal of the ACI, vol. 12, pages 281-293, 1941; Timms et al., “Use of Fly Ash in Concrete”, ASTM Proceedings, 1956; and Cabrera et al., “Design and Properties of High-Volume Fly Ash High-Performance Concrete”, American Concrete Institute, SP 186-2, p. 21-37, 1999. In most of these patents and publications, the fly ash utilized comprises any of those fly ashes which meet the requirements of ASTM (American Society for Testing and Materials) C 618, “Standard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete.”
It is also known that fly ash can be incorporated into controlled low-strength materials (often referred to as “CLSM”). In the publication “Controlled Low-Strength Materials”, reported by American Concrete Institute Committee 229, June 1999, there is provided a description of controlled low-strength materials along with certain ingredient mixtures used to produce CLSM. Controlled low-strength materials are broadly defined in this publication as self-compacted, cementitious materials used primarily as a backfill in place of compacted fill. Conventional CLSM mixtures usually consist of water, portland cement, fly ash, and fine or coarse aggregates. Some CLSM mixtures consist of water, portland cement and fly ash. However, CLSM is not to be considered as a type of low-strength concrete. This publication also defines CLSM as a material that results in a compressive strength of 8.3 MPa (1200 psi) or less at the conventional 28 day testing period (typically without compaction), and notes that most current CLSM applications require unconfined compressive strengths of 2.1 MPa (300 psi) or less at the conventional 28 day testing period in order to allow future excavation. This publication makes reference to certain examples of CLSM mixtures which include fly ash. U.S. Pat. Nos. 6,821,336 and 6,461,424 disclose the use of fly ashes in controlled low-strength materials, and U.S. Pat. Nos. 5,951,751 and 4,374,672 disclose the use of fly ashes which meet the requirements of ASTM C 618 in controlled low-strength materials.
It is known that fly ash is a voluminous by-product of coal burning electrical power generation plants, presenting a possible environmental disposal issue. Thus, the above patents and publications can provide an economically advantageous means of beneficial utilization of the fly ash waste by-product.
Activated carbon sorbent materials are now beginning to be utilized to capture mercury from industrial and manufacturing processes. For example, coal fueled electric power generating units are being retrofitted with additional equipment to inject activated carbon sorbents into the combustion gases at various points to capture mercury. In some cases, the activated carbon is injected into existing electrostatic precipitators or baghouses that also collect fly ash. This results in a commingled spent sorbent/fly ash mixture. In other cases, an additional baghouse is installed after the primary fly ash particulate collection device for the injection of sorbent to capture mercury while preserving the existing fly ash quality for beneficial use such as the uses in concrete and CLSM described above. Examples of the collection of mercury using sorbents can be found in U.S. Pat. Nos. 6,521,021, 6,451,094, 5,854,173, 4,889,698 and 4,273,747.
The above mercury collection processes result in new by-products that consist of primarily spent activated carbon sorbent, small amounts of mercury, and ultra fine fly ash that gets past the primary fly ash particulate control device. As mercury controls are added to power plants and other industrial processes, the amount of spent sorbent is expected to grow and consume space in landfill facilities.
Air dried concrete is considered a reasonably good electrical insulator, having a resistivity on the order of 106 ohm-cm, with oven dried concrete having a resistivity on the order of 1011 ohm-cm. Moist concrete, on the other hand is an electrolyte having a resistivity on the order of 104 ohm-cm, which leads to its classification as a semiconductor. Since the transmission of electrical charge in moist concrete occurs through the movement of dissolved ions in the electrolytic solution, higher cement content and higher water content result in lower resistivity. High water content, however, is not acceptable for structural concrete, since it also results in lowered compressive strength and density. It has been found that there is a direct relationship between the degree of hydration of the cement paste and resistivity, yielding a linear relationship between resistivity and compressive strength of cement paste and concrete. That is, resistivity increases as the compressive strength increases.
Electrically conductive concrete may be produced by placing electrically conductive fibers and/or particles in close contact with each other so that a conductive network may be formed throughout the concrete. In conductive concrete, the transmission of electrical charge occurs mainly through the conductive additives, rather than through the electrolytic solution created in moist concrete. Such additives as carbon fibers, steel fibers, steel shavings, and carbon black have been found to be effective in modifying the conductivity of concrete into which they are blended. For example, U.S. Pat. No. 3,962,142 teaches the use of calcined oil coke and acetylene black aggregates in conductive concrete having satisfactory mechanical strength, while U.S. Pat. No. 5,908,584 teaches a mixture of graphite, amorphous carbon, and sand, comprising 25 to 75% of a cementitious composite useful for conducting floors, heating elements, and ground connectors.
Electrically conductive concrete and controlled low-strength materials would be advantageous where lowered electrical resistance may be sought, such as for use in structures where it is necessary to protect electrical equipment from lightning strikes. Accordingly, a means to reduce the electrical resistance of concrete or controlled low-strength materials, or to increase the conductivity thereof, is of interest in the building industry, for example. Further, since carbon sorbent materials are now beginning to become available as a waste product, and carbon is known to be highly conductive, the use of spent carbon sorbent as an additive to concrete or controlled low-strength materials to lower electrical resistance has now been investigated.